This Instructable describes how to replace the fan inside a standard PC power supply. You may want to do this because the fan is defective, or to install a different type of fan, for example, an illuminated one.
In my case, I decided to replace the fan because my cheap power supply's fan started making enough noise to drive me to distraction...WARNINGS
Power supplies have dangerous voltages inside, even when completely disconnected. Capacitors on the line side usually retain their full charge even when unplugged, and can inflict a painful or even lethal shock. Please proceed only if you know what you are doing.
Disassembling the power supply will void its warranty.
Opening your PC may void its warranty, though I haven't come across such a computer so far. Also, fiddling with the insides may damage other components, so go ahead only if you are sure of yourself.
: Corrected explanation of fan dimensions. Thanks to KanyonKris
for the correction (see comment at bottom)